Mercury has long been known as a potential health and environmental hazard. Environmental standards for its emissions from coal fired power plants, petroleum and chemical refineries, incinerators, metal extraction operations, and other mercury emitting facilities are becoming increasingly demanding. New regulations are currently under development to reduce the permissible levels of mercury emissions from such facilities. Technologies are under development to meet this challenge. One such technology utilizes activated carbon to control mercury emissions from coal fired power plants. However, cost estimates show that commercialization of this technology would result in a five percent increase in electricity prices and that 95 percent of this increase is due to the cost of activated carbon.
It is an objective of the present invention to find lower cost carbon materials to use to control mercury emissions. Our studies have revealed that unburned carbons in or from fly ash, wood ash, and other charred carbonaceous materials are effective adsorbents for mercury. These carbon sources will be collectively referred to herein as "fly ash". These carbons can be used as a substitute for activated carbon. Compared with activated carbons, the unburned carbons from ash are much less expensive because they are usually combustion by-products. While fly ash may only contain a small percentage of carbon, the technologies to upgrade the ash to a higher carbon content, are cost effective. Surface treatment of the carbon, e.g. surface oxidation, will also enhance its adsorption for mercury.
We have found that the unburned carbons have a similar or higher adsorption capacity for mercury than activated carbon. The reason for this may be due to the pore structure of the carbons and the adsorption characteristics of mercury. In an activated carbon injection system for example, the dry activated carbon is carried by high speed air from an air compressor and sprayed into the flue gas duct, upstream of the particulate collection device. The carbon injection rate has been reported to be 1,000 to 10,000 times the mercury emission rate, with a carbon concentration of 30-80 mg/m.sup.3 in flue gas. Factors that affect carbon performance in a carbon injection emission control system include temperature, relative humidity, mercury concentration and other constituents of the flue gas. In the adsorption process, the carbon-mercury contact time is very short and adsorption equilibrium may be difficult to reach. It is anticipated that since the many of the pores in the activated carbon are in the micro-pore range, i.e. less than 2 nanometers, that activated carbon has less chance to adsorb mercury because of diffusion limitations. As a result, the potential adsorption capacity of the activated carbon will not be effectively utilized.
In the case of unburned carbons, the majority of the pores are in the macropore range, i.e. greater than 50 nanometers. Although these carbons have much lower surface area compared to activated carbons (e.g., 15-200 m.sup.2 /g for one of the fly ash carbons, 500 to 1,000 m.sup.2 /g for many activated carbons), they may adsorb mercury as effectively as commercial activated carbon in a carbon injection system. This assumes macro-pores to be more important than micro-pores and a minimum sorbent-to-gas ratio to be required in this situation. A minimum solid-to-gas ratio is usually required to ensure the adsorbate molecules, mercury in this case, in the gas phase have a reasonable chance to collide with adsorbent particles.
Compared with activated carbon, unburned carbon is generally low cost with a reasonable adsorptive capacity. Unburned carbon has more macro-pores, which allows the fast adsorption and easy regeneration after loaded. Moreover, the trace and minor elements or compounds present in the unburned carbons may enhance the adsorption of mercury. The primary use of unburned carbon to remove mercury is in the flue gas from coal-fired power plants. However, it can be used to remove mercury from incineration flue gas, natural gas and the ventilation air from chloralkali processes.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.